jumping in a bit late but I'd agree it depends on relative humidity. Summer / spring in Alberta I'd say you're good for 6 hours minimum before any real problem. On an SKS I've found rust starts in the gas tube or the piston first, barrels being different steel. I carry a bottle of Windex in my truck, if I'm going to be somewhere I can't clean properly I give the gun a few shots of that and even without a patch they seem good for a day or so. But yes, clean within a reasonable time.